1. Field of the Invention
The present invention relates to an electronic endoscope system in which an endoscope image is reproduced as a full color image on a TV monitor, and, in particular, to such an electronic endoscope system with a color-balance alteration process, which is constituted such that, for example, the endoscope image can be reproduced on the TV monitor as if it were sprayed with a dye-solution.
2. Description of the Related Art
As is well known, an electronic endoscope system includes a video scope that is inserted in an organ of a human body, and the video scope has a solid-state image sensor for capturing an organ image or endoscope image as a frame of image-pixel-signals. The electronic endoscope system also includes an image-signal processing unit for producing a video signal based on the frames of image-pixel-signals successively read from the solid-state image sensor, and a TV monitor for reproducing the endoscope image as a motion picture based on the video signal fed from the image-signal processing unit.
Recently, it is usual to manufacture an electronic endoscope system such that the endoscope image is reproduced as a full color motion picture on a TV monitor. Thus, a dye-spraying examination method was developed and has been used as a medical examination method in the medical field in which electronic endoscope systems are used. For example, when a subtle uneven surface of the mucous membrane of a stomach or a colon is examined, the dye-spraying medical method is utilized.
In particular, the mucous membrane surface of the stomach or the colon features a reddish orange tone as a whole, and thus it is very difficult to examine the subtle unevenness of the mucous membrane surface. In order that the subtle unevenness of the mucous membrane surface can be clearly and easily examined on a TV monitor, a bluish solution, such as an Indigo Carmine solution, is introduced into a forceps-insertion passage of the video scope, and is sprayed over the mucous membrane surface. The solution has a tendency toward gathering in fine recess areas on the mucous membrane surface, and it flows away from fine land areas on the mucous membrane surface. Namely, the fine recess areas on the mucous membrane surface are colored blue and clearly contrast with the reddish orange areas. Thus, it is possible to easily carry out an examination of the subtle unevenness of the mucous membrane surface.
However, there are various drawbacks in the dye-spraying medical examination method. For example, a dye must be harmless to a human body, and it is troublesome to develop harmless dyes. Also, the use of a dye-spraying medical examination method prolongs the medical examination time when using the electronic endoscope system, resulting in an increase in the patient's pain and discomfort. Further, once a dye-solution is sprayed, it is impossible to immediately reproduce an endoscope image without the sprayed dye-solution.
In order to resolve the above-mentioned problems, Japanese Laid-Open Patent Publication (KOKAI) No. 2001-25025 discloses an electronic endoscope system with a simulated dye-spraying process or color-balance alteration process for electronically processing an endoscope image as if it were sprayed with a blue-solution.
In particular, a full color endoscope image is formed based on three frames of three primary-color image-pixel-signals: a frame of red image-pixel-signals, a frame of green image-pixel-signals, and a frame of blue image-pixel-signals. In the color-balance alteration process, for example, nine red image-pixel-signals, forming a 3×3 matrix, are successively extracted from the frame of red image-pixel-signals, and a value of the central red image-pixel-signal is compared with an average of values of the eight surrounding circumferential red image-pixel-signals.
When the value of the central red image-pixel-signal is lower than the average of the values of the circumferential red image-pixel-signals, the central red image-pixel-signal derives from a fine recess area on a mucous membrane surface of, for example, a stomach. However, when the value of the central red image-pixel-signal is higher than the average of the values of the circumferential red image-pixel-signals, the central red image-pixel-signal derives from a fine land area on the mucous membrane surface of the stomach. The same is true for the green image-pixel-signals and the blue image-pixel-signals.
Accordingly, for example, if the frames of the three primary-color image-pixel-signals are processed such that the values of red and green image-pixel-signals, deriving from the fine recess areas, are lowered, an endoscope image can be reproduced as if it were sprayed with a bluish-solution.
In short, a fine recess area on the mucous membrane surface is detected by comparing the value of the central image-pixel-signal with the average of the eight values of the circumferential image-pixel-signals, and the values of central red and green image-pixel-signals, deriving from the fine recess area, are lowered when the fine recess area is detected.
Nevertheless, in the electronic endoscope system, as shown in the aforesaid KOKAI No. 2001-25025, there may be a case where the fine recess areas on the mucous membrane surface cannot be properly detected. For example, when the fine recess areas on the mucous membrane surface form a network of fine grooves, three aligned image-pixel-signals on the 3×3 matrix, including the central image-pixel signal, may lie on the fine groove. In this case, since the average of the eight values of the circumferential image-pixel-signals may be considerably lowered, it is not possible to properly detect the fine recess area corresponding to the central image-pixel-signal.
In another example, when a fine shallow recess area on the mucous membrane surface is adjacent to one or more fine deep recess area, and when the central image-pixel signal derives from the fine shallow recess area, the average of the eight values of the circumferential image-pixel-signals, may be considerably lowered due to the existence of the adjacent one or more fine deep recess areas. Similarly, it is not possible to properly detect the fine shallow recess area corresponding the central image-pixel-signal.